At the present time, electric wiring is generally used as a data transfer path for connecting CPU, a memory, input and output devices and the like to one another in a computer. An increase in operation speed of the computer in recent years, however, has heightened the importance of increasing the data transfer capacity. Further, regarding recent computers, extensive and intensive studies have been made wherein a plurality of processors are mounted in a computer and connected to each other or one another in a parallel form to constitute a network. In this case, increasing the capacity of the transfer path is a large task to be accomplished. Further, also in a communication apparatus, increased required throughput of an exchange has lead to a demand for increased capacity of transfer between boards in a communication apparatus or between communication apparatuses. The number of electric wirings has hitherto been increased to cope with the increase in the number of apparatuses connected and the increase in capacity. Increasing the number of electric wirings, however, has reached the limit.
In order to cope with the increase in data capacity, studies on an optical network technique using light instead of electricity have been energetically made. Among others, an optical switch which is one construction for realizing a data transfer path using light is particularly important.
In general, optical switches, such as an optical switch comprising a waveguide provided on an LiNbO.sub.3 substrate to utilize the electrooptic effect of LiNbO.sub.3 and an optical switch utilizing the thermooptic effect of a quartz waveguide, have been developed from the viewpoint of superior expandability of the scale.
In the LiNbO.sub.3 optical switch, however, a several tens of volts is necessary as the switching voltage, imposing a burden on a driver for driving the optical switch. Further, as described in R. Nagase et al., Journal of Lightwave Technology, Vol. 12 No. 9, 1994, pp. 1631-1639, the quartz waveguide type optical switch requires a power of one W per device, posing a problem of large power consumption.
An optical switch using a gate function of a semiconductor light amplifier has been proposed as an optical switch alternative to the above optical switches. In this optical switch, a signal output from each optical transmitter to an optical fiber is branched in a splitter and enters a semiconductor optical amplifier. The semiconductor optical amplifier functions as a gate for performing ON/OFF of light. Therefore, selection of any path for output followed by driving of only a semiconductor optical amplifier in this path permits an amplified signal to be transmitted through a combiner to an optical receiver in a desired path.
In the conventional optical switch, however, a splitter and a combiner are indispensable. A fiber fusing type coupler is used in the splitter and the combiner. It is known that loss unfavorably increases with increasing the number of branches. This is because the branch loss is essentially present in the branch section of the coupler.
For example, when an 8-input, 8-output optical switch is constructed, a signal power loss of 1 dB and a signal power loss of 8 dB (9 dB in total) are created respectively in the splitter section and the combiner section. That is, a total power loss as large as 18 dB (power loss in inlet+power loss in outlet) is created. Therefore, despite -he amplification in the semiconductor optical amplifiers the signal admitted into the receiver is very weak.
In particular, an increase in capacity of computers and communication networks in recent years has rendered an increase in scale and speed of the optical switch indispensable, necessitating increasing the light receiving level required for increasing the speed of the receiver. In the construction including a splitter and a combiner using a fiber fusing type coupler, it is very difficult to achieve the increase in transfer speed and scale. In fact, the processable signal is as low as several Gbit/s on 2.times.2, 4.times.4 level.
In order to reduce the above branch loss, Japanese Patent Laid-Open No. 291604/1987 discloses slant connection of one single mode waveguide to the side face of one multimode. In this construction, the branch loss is reduced by providing a tapered one single mode waveguide on the side face of one multimode waveguide, and it is difficult to connect a number of single mode waveguides to the multimode waveguide.